An above described method and an above described light pattern are known from WO 2004/083778 A1. There the method is used for scanning the surface profile of an object. From DE 10 2007 057 771 A1 a method and a device are known, which are mainly used for the volume determination of packages in packages sorting plants, wherein the packages are transported on conveying systems. The conveying system can comprise one or more conveyor belts or several conveyor rollers driven one after the other, on which the to be measured packages are conveyed. The device for measuring the height or the height profile of an object or a package is in this case arranged stationary and can for example comprise a bridge across the conveyor belt or the conveyor rollers, which is arranged transversally to the conveying direction and on which the individual components of the device are mounted. The device comprises further a linear picture sensor, which is arranged transversally to a conveying direction of the conveying system. Furthermore, at least one light source is provided, which is collinear to the linear picture sensor and is arranged laterally off-set thereto. The light source produces a distinct patterned light sequence across the width of the conveying system and the moving surface. This light sequence is reflected and recorded by the linear picture sensor. Thus, the light sequence is initially projected onto the conveyor belt transversally to the conveying direction. The linear picture sensor captures this light sequence from a position, laterally off-set in relation to the light source. Packages are moved by the conveyor belt through the light sequence. Here because of the off-set arrangement of the linear picture sensor, an off-set of the part of the light sequence, which is reflected by the package, is produced in a direction of the longitudinal extension of the light sequence, i.e. transversally to the conveying direction. This off-set or this displacement of the part of the light sequence, reflected by the package, is proportional to the height of the package, so that then, by means of the width of the displaced portion of the light sequence, the conveying speed and the size of the off-set, the volume of the package can be derived.
The light sequence is a spatial distinct bar arrangement, which is produced by a laser illumination device, which comprises a diffraction lens (DOE, diffractive optical element). The spatial distinct bar arrangement comprises a row of illuminated long bars and illuminated short bars. These are separated from each other by short empty spaces and long empty spaces. Thus, the bar arrangement can be compared with a bar code. The bar arrangement contains 199 code words, wherein one code word is determined by six elements. An element is an illuminated bar or an empty (not illuminated) space. The code words are arranged sequentially one after the other. The distinct pattern is determined by 78 words and is repeated three times across the width of the conveyor belt. Thus, within one of the three distinct patterns, each individual code word can be determined and identified, wherein then, also the position of the code word is known across the width of the conveyor belt. Thus, when the code word is off-set, the size of the off-set and thus the height of the package can be determined.
The problem is, that it has to be differentiated between long and short illuminated bars and between long and short empty (not illuminated) spaces. This can lead to problems in practice, when packages with different surfaces, e.g. light and dark surfaces, are sorted. Light surfaces reflect the light pattern stronger than dark surfaces, so that excessive irradiation produced. Excessive irradiation is characterised in that the correspondingly excessive illuminated bar seems to be longer on the picture captured by the linear picture sensor than a normal illuminated bar, which for example is reflected by a dark surface. Furthermore, when two excessive illuminated bars are arranged next to each other, the empty space can seem to be narrower. This phenomena can lead to a wrong determination or no determination of a code word.
Furthermore, picture sensors have a depth of focus, which can lead at the edge of areas of depth of sharpness to be out of focus. Being out of focus leads also to the phenomena, that an illuminated bar is reproduced wider than it is actually and correspondingly empty non-illuminated spaces between two represented or reproduced illuminated bars being out of focus seem to be narrower.
Furthermore, in principle a complete code word has to be read or determined. In code words, arranged sequentially one after the other and respectively consisting of six elements, a multitude of individual elements has to be read. When for example the reading is only started at the second element of a code word, then the residual elements of the first code word have to be read and all elements of the following code word, till a complete code word, in this case the second code word, is recognised.